Where f denotes mass shooting fatalities, pop is population in millions, trump is a dummy variable for Trump administration.

One can interpret this as follows: a Trump administration quarter is associated with 5.6 greater fatalities from mass shootings, or 22.4 on an annualized basis. (Over 1982q4-16q4, the average fatalities per quarter is 4.876). Inclusion of a deterministic time trend yields a negative coefficient on population, and a trump coefficient (4.464) significant at 6% msl.